1. Field of the Invention
The present invention relates to a solar cell and a fabrication method thereof, particularly a solar cell with a bypass function including the function of a bypass diode that protects the solar cell from reverse bias voltage generated when, for example, the solar cell module is shadowed, and a method of fabricating such a solar cell.
2. Description of the Background Art
A solar cell module has a plurality of solar cells combined in series or in parallel to obtain a predetermined output voltage and output current. In the event of some of the cells being shadowed, voltage generated by the other cells will be applied in the reverse direction.
When the reverse breakdown voltage of the shadowed cells is surpassed by the reverse bias voltage applied in the reverse direction, breakdown occurs at the relevant cells to result in a great flow of current. There is a possibility of short-circuit failure occurring in these cells. Eventually, the output property of the entire solar cell module is degraded.
In the case of a solar cell module for use in space, the shadow of a portion of the satellite or any structural member such as the antenna may fall on the solar cell module during control of the posture of the satellite. In the case of a solar cell module for terrestrial use, the cells may be shadowed by an adjacent building or by droppings of a bird or the like.
Consider the case where a shadow falls on the submodule of a solar cell module formed of a series of solar cells connected in parallel.
FIGS. 12A and 12B show a structure of a conventional solar cell module.
In a shunt mode where both ends of a solar cell module M are substantially short-circuited as shown in FIG. 12A, power V12 generated by a group of submodules 12 not shadowed is applied as a reverse bias voltage to the shadowed submodule 11. Therefore, V11=xe2x88x92V12, where V11 is the voltage of submodule 11.
In the case where an external power source VB is connected to solar cell module M as shown in FIG. 12B, the voltage of submodule 11 becomes V11=VBxe2x88x92V12. Positive charge is applied to the N electrode of shadowed submodule 11. When the reverse bias voltage thereof exceeds the reverse breakdown voltage of the solar cells forming submodule 11, breakdown occurs at that solar cell, resulting in the possibility of short-circuit failure. Accordingly, the output property of the shadowed module 11 is degraded, which in turn degrades the entire solar cell module M.
In order to prevent the disadvantage caused by such reverse bias voltages, a bypass diode may be attached for each solar cell or for every unit of specific modules. Alternatively, the so-called diode integrated solar cell having a bypass diode integrated with the solar cell is used.
Furthermore, a solar cell added with the bypass diode function is known. An example of a structure of a conventional solar cell with the bypass diode function will be described here with reference to the drawings.
FIG. 13 is a perspective view of a high-efficiency bypass diode function added solar cell with a reflectionless surface construction.
Referring to FIG. 13, the conventional solar cell includes a silicon substrate 1 of a first conductivity type such as the P type, a region of a second conductivity type such as the N type formed at the light receiving plane of substrate 1 to efficiently collect carriers generated by light energy, a P+ region 3 formed at the bottom plane of substrate 1 for the back surface field (BSF) effect, an island-like P+ region 4 provided at a portion of the light receiving plane of substrate 1 for bypass, an N electrode 7 provided at the surface of the N type region to obtain generated electricity efficiently, an anti-reflection film 8 covering substantially the entire plane of the N type region except for an N electrode connection portion not shown to reduce the surface reflection of incident light, and a P electrode 6 covering substantially the entirety of the bottom plane of P+ region 4 to reflect light of a long wavelength passing through the bottom plane and to produce the generated electricity. The solar cell also includes a gridded reflectionless surface construction 13 provided at the light receiving side to reduce surface reflection, formed of a grid configuration having a plurality of recesses in the shape of upside down pyramids, and an oxide film layer 9 (not shown) on the N+ diffusion layer to reduce recombination of carriers at the surface. An oxide film layer 5 is provided on P+ diffusion layer 3 to reduce recombination of carriers at the bottom face of P type silicon substrate 1. N+ diffusion layer 2 and surface electrode 7 are connected via an opening not shown in the oxide film layer. P+ diffusion layer 3 and back electrode 6 are connected via an opening in oxide film layer 5.
In a solar cell of the above-described structure, reflectionless surface construction 13 formed at the light receiving side serves to multiple-reflect the incident light to increase the quantity of light arriving inside the solar cell. The generated power depends upon the formation thereof. Therefore, the setting of the method of forming this configuration is extremely critical. In the present solar cell, reflectionless surface construction 13 is formed all over except for the region where surface electrode 7 is formed and the region of the ends of the solar cell.
FIG. 14 is a top view of the solar cell of FIG. 13, showing a circular region 10 for the P+ diffusion region.
Referring to FIG. 14, the size and shape of each grid unit are identical. Therefore, the interval between each grid unit of the reflectionless surface construction is all identical.
A method of fabricating such a conventional solar cell of the above structure will be described hereinafter.
FIGS. 15A-15G and FIGS. 16A-16E are sectional views of the solar cell of FIG. 13 to describe a fabrication method thereof.
The reflectionless surface construction is fabricated according to the steps of FIGS. 15A-15G.
Referring to FIG. 15A, a silicon substrate 1 of plane orientation (100) is prepared.
Referring to FIG. 15B, oxide film 9 is formed by thermal oxidation or CVD at the surface of silicon substrate 1.
Referring to FIG. 15C, a resist 15 is applied on oxide film 9.
Referring to FIG. 15D, a predetermined pattern of reflectionless surface construction 13 and the portion of the alignment mark not shown are exposed and developed at the light receiving side. As a result, the pattern to form the reflectionless surface construction and the pattern of the alignment mark are formed by means of resist 15 on oxide film 9.
Referring to FIG. 15E, oxide film 9 of the unrequired region is removed by etching or the like. Then, resist 15 is removed. Thus, the pattern of the reflectionless surface construction by oxide film 9 and the pattern of the alignment mark not shown are provided on silicon substrate 1. At this stage, the distance d between the grid units is determined.
Referring to FIG. 15F, etching is applied for a predetermined time using an etching solution of a predetermined temperature and concentration such as a high temperature alkaline solution. For silicon substrate 1, the etching rate with respect to the chemical substance differs for each crystal plane. By means of anisotropic etching thereof, a fine reflectionless surface construction 13 can be formed. At this stage, the alignment mark region takes a recess configuration.
Referring to FIG. 15G, oxide film 9 is removed, completing the formation of reflectionless surface construction 13 and the alignment mark not shown at the light receiving side of silicon substrate 1.
The structure of providing the bypass function to the solar cell of FIG. 13 is achieved by the fabrication steps of FIG. 16A-16E corresponding to sectional views of the solar cell.
Referring to FIG. 16A, P type silicon substrate 1 having reflectionless surface construction 13 and an alignment mark region not shown formed at the light receiving side obtained by the foregoing method is prepared.
Referring to FIG. 16B, oxide film 9 is formed by thermal oxidation or the like at the surface of P type silicon substrate 1.
Referring to FIG. 16C, oxide film 9 at the backside is removed, and a plurality of openings 14 are formed by photolithography or the like in oxide film 9 at the surface side. The plurality of openings 14 correspond to island-like P+ region 4 that will be formed afterwards.
P+ impurities having an impurity concentration of approximately 1xc3x971020 cmxe2x88x923, for example, are diffused into this wafer. Then, by removing oxide film 9 from the surface and the side, the wafer as shown in FIG. 16D is obtained. This wafer has a plurality of island-like P+ regions 4 formed at the surface by boron diffusion, and P+ region 3 formed all over the backside in the case of the all-surface BSF type.
Referring to FIG. 16E, N type region 2 is formed at the top surface and side face by thermal diffusion or the like. Since island-like P+ region 4 is protected by the boron glass remaining at the surface, the island of P+ region 4 is left in N region 2. The outer circumferential portion of island-like P+ region 4 is identical to circular region 10 at the surface of texture configuration 13 shown in FIG. 14.
Then, using the alignment mark region, the surface electrode is formed by photolithography or the like so that the island of P+ region 4 is formed between the grid units, and an anti-reflection film or the like is formed.
FIG. 17 is a perspective view of another conventional solar cell showing another structure. This solar cell with the bypass diode function does not have a reflectionless surface construction.
FIG. 18 is a top view of the solar cell of FIG. 17.
The solar cell of FIGS. 17 and 18 differs from the solar cell described with reference to FIG. 13 in the absence of reflectionless surface construction 13.
The structure to provide the solar cell of FIG. 17 with the bypass function is fabricated by the steps of FIGS. 19A-19G corresponding to sectional views thereof.
Referring to FIG. 19A, P type silicon substrate 1 is prepared.
Oxide film 9 is formed by thermal oxidation or the like at the surface of P type silicon substrate 1, resulting in the structure of FIG. 19B.
Referring to FIG. 19C, an opening 20 is formed by photolithography or the like in oxide film 9 corresponding to the alignment mark region provided at the outer side to the cell pattern at the surface of P type silicon substrate 1.
Then, the wafer is subjected to etching for a short period of time using an alkaline solution, whereby the alignment mark region of P type silicon substrate 1 is etched to a recess configuration, resulting in the wafer of FIG. 19D with an alignment mark of a recess 21.
Then, oxide film 9 at the backside is removed, and a plurality of openings 14 are formed by photolithography in oxide film 9 located at the surface to result in FIG. 19E. Openings 14 correspond to island-like P+ regions 4 that will be formed afterwards. P+ type impurities of approximately 1xc3x971020 cmxe2x88x923 in impurity concentration are diffused to the wafer.
By removing oxide film 9 from the surface and the side face, a plurality of island-like P+ regions 4 are formed at the surface by, for example, boron diffusion, and P+ region 3 is formed all over the backside in the case of the back surface field reflector (BSFR) type, resulting in the wafer shown in FIG. 19F.
Referring to FIG. 19G, N type region 2 is formed by thermal diffusion or the like at the surface and the side face. Island-like P+ region 4 and the surface of P+ region 3 at the backside have boron glass remaining thereon so as to be protected. Therefore, the island of P+ region 4 remains in N region 2 at the light receiving side.
Island-like P+ region 4 is formed between the surface electrode grids by photolithography or the like using alignment mark recess 21 formed as described above. The process steps of FIGS. 19C and 19D are necessary since P+ region 4 cannot be formed between the surface electrode grids if there is no alignment mark recess 21. Then, anti-reflection film 8 and the like are formed.
A plurality of such solar cells are connected in parallel as shown in FIG. 12A to be used as the general solar cell module M under the predetermined voltage and current. If reverse bias voltage is applied to the solar cell of the above-described structure, the P+N junction of the region of the second conductivity type (for example N type) at the light receiving side and the region of the first conductivity type (for example, P type) of high concentration formed in contact with the second conductivity type region is reversely biased. Zener effect easily occurs at this portion by the PN junction established by the first conductivity type substrate and the second conductivity type diffusion layer to cause breakdown.
When current is generated in the reverse direction at this region where a relatively small reverse bias voltage is applied, increase in the reverse bias voltage causes the Zener effect. This will prevent the reverse bias voltage from being applied to the body of the solar cell. This will be described with reference to an equivalent circuit diagram of a solar cell with a bias function shown in FIG. 20.
FIG. 20 corresponds to the structure in which an NP+ diode is connected in parallel to the solar cell formed of an NP junction. When reverse bias voltage is applied, the current of the NP+ diode that has a great leakage current in the opposite direction flows. Therefore, the solar cell is protected from breakdown. Island-like P+ type region 4 has a structure in which breakdown occurs by the Zener effect caused by the PN junction formed between island-like P+ region 4 and N type region 2 by increasing the impurity concentration higher than that of P type silicon substrate 1. The impurity concentration of P+ region 4 is to be set to at least 1xc3x971018 cmxe2x88x923 to cause the Zener effect. It is to be noted that the output of the solar cell will be degraded as the total area of island-like P+ region 4 becomes larger. Therefore, designing is necessary to set the total area of P region 4 as small as possible within the range where the solar cell is not fractured while causing Zener breakdown.
Solar cells with island-like bypass regions provided are disclosed in Japanese Patent Laying-Open Nos. 5-110121 and 10-163511.
However, the conventional high-efficiency bypass function added solar cells with the reflectionless surface construction had the problem that the bypass function could not be easily achieved since reverse current of a satisfactory level could not be obtained. Although the reverse current can be increased by enlarging the area of the P+ region formed on the reflectionless surface construction to provide the bypass function, the N type diffusion region at the light receiving side will have to be reduced corresponding to the increase of the P+ region. This means that the output of the solar cell is degraded. Thus, increasing the reverse current without changing the area of the P+ region formed on the reflectionless surface construction was an issue.
The conventional bypass function added solar cell absent of the reflectionless surface construction has a greater number of fabrication processing steps than those of the solar cell that does not have the bypass function. There was also the problem that the bypass function could not be easily achieved since reverse current of a satisfactory level could not be obtained.
For the BSFR type solar cell that has, for example, a P type diffusion region at the backside or the back surface reflector (BSR) type solar cell (not shown) that does not have a P type diffusion region, the alignment mark region is indispensable to form P+ regions 4 between the N electrode grids shown in FIG. 17. The photolithography step to form such alignment marks is necessary. Therefore, the fabrication process is increased significantly than the process of the solar cell that does not have the bypass function.
An object of the present invention is to provide a structure and a fabrication method of a high-efficiency bypass function added solar cell with a reflectionless surface construction to increase reverse current without altering the area of the P+ type region formed on the reflectionless surface structure, overcoming the problems of the conventional art.
Another object of the present invention is to provide a structure and a fabrication method of a bypass function added solar cell absent of a reflectionless surface construction, having a reverse current greater than that of the conventional case without hardly increasing the fabrication process.
According to the present invention, the above objects can be achieved by a solar cell set forth in the following.
According to an aspect of the present invention, a solar cell includes a substrate of a first conductivity type, a region of a second conductivity type formed at a light receiving side of the substrate, an electrode formed at the region of the second conductivity type, and a region of higher concentration than the substrate, arranged in contact with both the substrate and the region of the second conductivity type, and not in contact with the electrode. The substrate includes a reflectionless surface construction and a planar portion. The reflectionless surface structure has a plurality of grid configurations, and includes a first grid portion and a second grid portion having a grid configuration differing in size from that of the first grid portion. The region of higher concentration than the substrate is formed at the first grid portion.
According to another aspect of the present invention, the size of each grid configuration of the first grid portion of the reflectionless surface construction is greater than each grid configuration of the second grid portion.
According to a further aspect of the present invention, the first and second grid portions of the reflectionless surface construction have different grid configurations.
According to still another aspect of the present invention, the distance between the grids of the first grid portion differs from the distance between the grids of the second grid portion in the reflectionless surface construction.
According to a still further aspect of the present invention, the distance between the grids of the first grid portion is smaller than the distance between the grids of the second grid portion in the reflectionless surface structure.
According to yet a further aspect of the present invention, a solar cell includes a substrate of a first conductivity type, a region of a second conductivity type formed at a light receiving side of the substrate, an electrode formed at the region of the second conductivity type, and a region of higher concentration than the substrate, arranged in contact with both the substrate and the region of the second conductivity type, and not in contact with the electrode. The region of higher concentration than the substrate is formed in a recess.
According to yet another aspect of the present invention, a method of fabricating a solar cell is provided. The solar cell of the present aspect includes a substrate of a first conductivity type, a region of a second conductivity type formed at a light receiving side of the substrate, an electrode formed at the region of the second conductivity type, and a region of higher concentration than the substrate, arranged in contact with both the substrate and the region of the second conductivity type, and not in contact with the electrode. The substrate includes a reflectionless surface construction and a planar portion at the light receiving side. The reflectionless surface construction has a plurality of grid configurations, and is formed of a first grid portion and a second grid portion having a grid configuration differing in size than that of the first grid portion. The region of higher concentration than the substrate is formed at the first grid portion. The fabrication method includes the step of forming a masking pattern on the substrate using an oxide film, and etching the substrate with the masking pattern as a mask. The line width of the masking pattern located at the second grid portion is larger than the line width of the masking pattern located at the first grid portion.
According to yet a still further aspect of the present invention, a fabrication method of a solar cell is provided. The solar cell of the present aspect includes a substrate of a first conductivity type, a region of a second conductivity type formed at a light receiving plane of the substrate, an electrode formed at the region of the second conductivity type, and a region of higher concentration than the substrate, arranged in contact with both the substrate and the region of the second conductivity type and not in contact with the electrode. The region of higher concentration than the substrate is formed in a recess. The fabrication method includes the steps of etching the portion of a window of a patterned surface oxide film using an alkaline solution to form a recess, and forming the region of higher concentration by diffusing impurities with the surface oxide film as a diffusion mask.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.